1. Technical Field
The present disclosure relates generally to an apparatus for driving an induction motor.
2. Related Art
As a motor drive system for industrial applications, a system by which an induction motor or a synchronous motor is variable-speed driven by using an inverter apparatus is widely used. For example, in railway vehicles, generally one to four induction motors are driven by a single inverter apparatus.
A railway vehicle typically accelerates from a deactivated state by starting the inverter apparatus, transitions to a coasting state in which the vehicle runs through inertia while the inverter apparatus is deactivated, and then accelerates or decelerates from the coasting state by restarting the inverter apparatus.
In the coasting state, the inverter apparatus is deactivated, so that no voltage is applied from the inverter apparatus to the induction motor. In principle, however, the magnetic flux in the induction motor does not disappear immediately but remains after the inverter apparatus is deactivated. The magnetic flux that remains in the induction motor when the inverter apparatus is deactivated is referred to as “the residual magnetic flux”. When there is the residual magnetic flux, an induced voltage proportional to the product of the residual magnetic flux and the rotation speed is produced in the induction motor.
In the state in which the inverter apparatus is deactivated, the energy of the residual magnetic flux is consumed by the resistance of the rotor of the induction motor, and the residual magnetic flux decays in accordance with a time constant determined by the values of resistance and inductance of the induction motor. In the case of an induction motor for railway vehicles, the time constant that determines the decay time of the residual magnetic flux is on the order of 200 to 600 ms.
The time it takes for the residual magnetic flux to decay to zero is approximately three times the time constant (namely, 600 to 1800 ms), considering the fact that the step response of a first-order lag system reaches approximately 95% of a target value in three times the time constant. The time constant with which the residual magnetic flux decays is on an increasing trend because of the increasingly widespread use of induction motors with smaller resistance values as part of the recent energy-saving measures.
While the duration of time for which the railway vehicle is coasted depends on the type of the vehicle and the method of operation, such as whether it is by manual operation or an automatic operation by an ATO (Automatic Train Operation) apparatus, the inverter apparatus may in some cases be restarted by ending the coasting operation before the residual magnetic flux decays to zero.
When the inverter apparatus is started in the state in which the residual magnetic flux remains, excessive current may flow depending on the phase of the induced voltage due to the residual magnetic flux and the voltage outputted from the inverter apparatus. This is not desirable from the viewpoint of protection of the inverter apparatus. This may further lead to transient torque variation, thereby adversely affecting the ride.
When speed sensor-less vector control is implemented such that, instead of detecting the rotation speed of the induction motor by using a speed sensor, the induction motor is driven by estimating the rotation speed from the current flow through the induction motor, speed estimation failure may be caused by the induced voltage as disturbance due to the residual magnetic flux.
The current may be suppressed by controlling the phase of the voltage outputted from the inverter apparatus if the phase of the residual magnetic flux can be accurately grasped. However, the addition of a sensor for detecting the phase of the magnetic flux, for example, results in an increase in manufacturing cost or maintenance cost. Further, when a method of estimating the phase of the magnetic flux is employed, the residual magnetic flux may not be accurately detected because of the influence of manufacturing errors in the resistance value or inductance value of the induction motor, variations in the constant due to temperature shifts and the like.
Because the residual magnetic flux decays in accordance with the time constant determined by the resistance value and inductance value of the induction motor as mentioned above, a method may be employed by which the inverter apparatus is restarted after a sufficient time. This method, however, limits the operation of the inverter apparatus and is therefore not preferable.
For the above reasons, it is preferable that the residual magnetic flux has decayed to zero when the inverter apparatus is restarted. Several methods for causing the decay of the residual magnetic flux have been proposed.
JP Patent Publication (Kokai) No. 2008-113501 A describes a method by which, when deactivating the inverter apparatus, the inverter apparatus is operated for a certain time by switching a voltage command for the inverter apparatus from a normal operation command to a command for causing the decay of the residual magnetic flux.
JP Patent Publication (Kokai) No. H10-66386 A (1998) describes a method by which, when deactivating the inverter apparatus, all of upper arms or lower arms of the inverter apparatus are simultaneously turned on so as to form a short circuit between the inverter apparatus and the induction motor.